1. Field of the Invention
The present invention relates to a method for an optical measuring of an OPC structure, in particular for measuring an OPC structure associated with a predetermined structure on a photo mask.
2. Description of the Related Art
The trend in semiconductor manufacturing goes more and more toward smaller and smallest structures. The common method here is the illumination of wavers using photomasks by means of light (e.g. using visual wavelengths or wavelengths in the UV range), ion beams, electron beams, x-rays, or other methods to be mapped (lithography). At that, the structures to be mapped, like e.g. thin conductive traces or small contacts, are often in the range of or even smaller than the used wavelengths, which inevitably leads to mapping errors. The limits for mapping for lithography methods which use visible light range from a structure size of about 350 nm to 400 nm, and the mapping limit for lithography methods which use UV light range from about 250 nm to 300 nm. In particular, due to the limited resolution, corners of structures or line ends are mapped strongly rounded off onto the waver.
In order to achieve a better pattern fidelity compared to the original design or layout, the above-mentioned critical locations (corners, line ends) on the photomask are provided with OPC structures or OPC-similar structures (OPC=Optical Proximity Correction).
In connection with the present description, the term OPC structure refers to any structure or any element which is added to a photomask in order to guarantee or support, respectively, the true mapping of the mask onto a substrate.
The OPC structures serve to change the structures actually to be generated on the photomask in a deliberate way in order to achieve a better mapping on the waver, i.e. for example less rounded at the corners.
With reference to FIG. 1, in the following two examples for the layout of OPC structures for the generation of photomasks are described. In FIG. 1A a section 100 of a layout is shown including a portion of a first structure 102. In order to prevent a roundoff in the area of the corner 104 due to the mapping of the layout 100 onto the photomask, an OPC structure 106 is provided there protruding beyond the horizontal edge 108 and the vertical edge 110 of the structure 102 in the area of the corner 104. In the example shown in FIG. 1A for a layout, the OPC structure 106 is substantially square. The OPC structure at the corner 104 shown in FIG. 1A is also referred to as corner serif.
In FIG. 1B a section 200 of a layout is shown together with a portion of a second structure 202. The second structure 202 is a line, and in the section 200 of the layout the line end of the structure 202 is illustrated. The structure 202 includes two parallel vertical edges 204 and 206 and a horizontal edge 208 connected to the vertical edges 204 and 206 in the area of a first corner 210 and in the area of a second corner. When transmitting the layout 200 onto a photomask, a similar problem results as in the transmission of the layouts described with reference to FIG. 1A, i.e. that the structure generated on the photomask in the area of the corners 210 and 212 is rounded off, so that also here, similar to FIG. 1A, an OPC structure needs to be provided in the area of the corners 210 and 212. In FIG. 1B two OPC structures 214 and 216 are arranged in the area of the corners 210 and 212, respectively, wherein the OPC structures respectively protrude beyond corners 204 and 208 and 206 and 208, respectively. As in FIG. 1A, also here the OPC structures are basically of a square nature. The structure shown in FIG. 1B is also referred to as line end serifs. A special case of theses OPC structures in which the serifs are adjacent to each other at the line end is also referred to as a hammerhead.
Conditional on the function, the OPC structures 106, 214, and 216 illustrated in FIGS. 1A and 1B are very small (approx. 200 nm and smaller).
Instead of the structures described in FIG. 1 also other structures and elements are possible, e.g. so-called jogs or scatterbars, in order to improve the edge quality.
In the conventional quality testing and quality assurance of photomasks for example generated using the layouts as they were described with reference to FIGS. 1A and 1B using optical microscopy, these small dimensions of the OPC structures represent a special challenge. Further, due to the high number of OPC structures in different spatial orientations on only one photomask a demand exists for an automatic method for the recognition and measuring of these structures.